With the recent reduction in weight, miniaturization, and increase in density of electronic products, demands for various printed circuit boards have been increasing. In particular, demands for flexible printed circuit boards (also referred to as “FPCs”) have been increasing. In general, a flexible printed circuit board has a structure in which a circuit composed of a metal foil is disposed on an insulating film.
A flexible metal-clad laminate constituting the flexible printed circuit board is usually produced by a method in which a metal foil is laminated by thermocompression bonding on a surface of a substrate made of a flexible insulating film with an adhesive material therebetween. As the insulating film, a polyimide film or the like is preferably used. As the adhesive material, a thermosetting adhesive, such as an epoxy or acrylic adhesive, is usually used (hereinafter, the FPC including such a thermosetting adhesive is also referred to as a “triple-layer FPC”).
The thermosetting adhesive has an advantage in that bonding can be performed at relatively low temperatures. However, as the requirements for characteristics, such as heat resistance, flexibility, and electrical reliability, become more stringent, triple-layer FPCs including the thermosetting adhesive will have difficulties in meeting such requirements. On the other hand, a FPC in which a metal layer is directly disposed on an insulating film or in which a thermoplastic polyimide is used for an adhesive layer (hereinafter also referred to as a “double-layer FPC”) has been proposed. The double-layer FPC has superior characteristics to those of the triple-layer FPC, and demands for the double-layer FPC are expected to increase.
Examples of the method for producing a flexible metal-clad laminate used for a double-layer FPC include a casting method in which a solution of a polyamic acid, i.e., a polyimide precursor, is cast and applied onto a metal foil, and then imidization is performed; a metallizing method in which a metal layer is directly disposed on a polyimide film by sputtering or plating; and a laminating method in which a polyimide film and a metal foil are laminated with a thermoplastic polyimide therebetween. Among these, the laminating method is superior because the laminating method can be used in the wider range of thickness of the metal foil than the casting method, and the equipment cost of the laminating method is lower than that of the metallizing method. As the lamination apparatus, a thermal roll laminator and a double belt press, or the like in which lamination is continuously performed while unreeling rolls of materials, is used. Among these, in view of productivity, the thermal roll laminating method can be more preferably used.
When the conventional triple-layer FPC is produced by the laminating method, since a thermosetting resin is used for the adhesive layer, lamination can be performed at a temperature less than 200° C. (refer to Japanese Unexamined Patent Application Publication No. 9-199830). In contrast, in the double-layer FPC, since a thermoplastic polyimide is used as an adhesive layer, in order to allow the thermoplastic polyimide to exhibit thermal fusibility, heat must be applied at a high temperature of 200° C. or more, and in some cases, about 400° C. Consequently, residual strain occurs in the flexible metal-clad laminate produced by lamination, resulting in changes in dimensions when interconnections are formed by etching and when solder reflow is performed in order to mount components. Therefore, in particular, when a flexible metal-clad laminate is produced by the laminating method, problems may be caused by changes in dimensions in the step of laminating a metal foil, the step of etching the metal foil, and the subsequent heating step.
Recently, in order to achieve miniaturization and weight reduction of electronic apparatuses, microfabricated wirings have been provided and miniaturized components having higher densities have been mounted on the boards. Therefore, if the change in dimensions increases after microfabricated wirings are formed, deviations occur from the component mounting positions originally designed, and thus the components are not connected satisfactorily to the boards.
Under these circumstances, attempts have been made to suppress the change in dimensions by controlling lamination pressure or by controlling tension of an adhesive film (refer to Japanese Unexamined Patent Application Publication No. 2002-326308 or 2002-326280). Although the change in dimensions is improved by such means, the improvement is not sufficient, and further improvement in the change in dimensions is desired.